Due to their image forming principles, liquid crystal display devices are required to have polarizing plates on the surface of the liquid crystal panels. The function of the polarizing plate is to absorb one of polarized light components orthogonal to each other (so-called P-polarized light wave and S-polarized light wave) and let the other of the polarized light components pass through the polarizing plate.
Hitherto, dichroic polarizing plates obtained by adding iodine-based or dye-based polymeric organic substances in films have often been used as such polarizing plates. As a common method for producing such polarizing plates, a method of staining a polyvinyl alcohol-based film with a dichroic material such as iodine, then allowing the film to undergo cross-linking using a cross-linking agent, and uniaxially stretching the film is used. Because dichroic polarizing plates are produced by stretching as described above, there is a general tendency that the dichroic polarizing plates easily shrink. Furthermore, because polyvinyl alcohol-based films use hydrophilic polymers, the polyvinyl alcohol-based films are highly likely to deform, particularly under humidified conditions. Moreover, because films are used in the first place, mechanical strength as devices is poor. In order to avoid this, a method of bonding a transparent protective film is employed.
In recent years, liquid crystal display devices have come to have a broadened range of applications and have become highly functionalized. Along with this, individual devices constituting the liquid crystal display devices are required to have a high reliability and durability. For example, in the case of liquid crystal display devices such as transmissive liquid crystal projectors that use light sources having a high light quantity, polarizing plates receive strong radiating rays. Therefore, the polarizing plates used in these devices are required to have an excellent heat resistance. However, because such film-based polarizing plates as described above are organic matters, there is naturally a limitation to increasing this property.
In the United States, an inorganic polarizing plate having a high heat resistance is available from Corning Incorporated under a product name POLARCOR. This polarizing plate has a structure of silver particles being dispersed in glass, and does not use an organic substance such as a film. Its principle is to utilize plasma resonance of island-like particles. That is, this principle is to utilize light absorption by surface plasma resonance that occurs when light becomes incident to island-like particles of a noble metal or a transition metal, and the absorption wavelength is affected by the shape of the particles and a surrounding dielectric constant. Here, when the shape of the island-like particles is elliptic, different resonance wavelengths are obtained in the longer-axis direction and the shorter-axis direction. This provides a polarizing property. Specifically, a polarizing property of a polarized light component parallel with the longer axis being absorbed along the longer wavelength side and a polarized light component parallel with the shorter axis being let to pass is provided. However, POLARCOR provides the polarizing property for a wavelength range close to the infrared range, but does not cover the visible light range required in liquid crystal display devices. This is due to the physical properties of silver used as the island-like particles.
PTL 1 describes an UV polarizing plate that is obtained by applying the principle described above and depositing particles in glass by thermal reduction, and presents a specific example in which silver is used as metal particles. In this case, it is considered that absorption along the shorter-axis direction is utilized conversely to POLARCOR described above. As illustrated in FIG. 1, although the function as the polarizing plate is effective even near 400 nm, the extinction ratio is low and the absorbable range is very narrow. Therefore, even if POLARCOR and the technique of PTL 1 are combined, a polarizing plate that can cover the whole visible light range cannot be obtained.
NPL 1 describes a theoretical analysis of an inorganic polarizing plate utilizing plasma resonance of island-like metal particles. This document describes that the resonance wavelength of aluminum particles is shorter than the resonance wavelength of silver particles by about 200 nm, and hence that there is a possibility that use of aluminum particles may make it possible to produce a polarizing plate that covers the visible light range.
PTL 2 describes some methods for producing polarizing plates using aluminum particles. It is described that silicate-based glass is not suitable as a substrate because aluminum and the glass react with each other, but that calcium aluminoborate glass is suitable (paragraphs [0018] and [0019]). However, glass using a silicate is widespread as optical glass, and highly reliable commercial products of this glass are available at low prices. Therefore, it is not economically favorable if this glass is not suitable. A method for forming island-like particles by etching a resist pattern is also described (paragraphs [0037] and [0038]). Typically, a polarizing plate used in a projector is required to have a size of about some centimeters and a high extinction ratio. Hence, for obtaining a polarizing plate for visible light, the resist pattern size needs to be a size of some tens of nanometers, which is sufficiently shorter than the visible light wavelength, and patterns need to be formed at a high density in order to obtain a high extinction ratio. Furthermore, for use in a projector, patterns covering a large area need to be formed. However, the described method of applying formation of high-density minute patterns by lithography requires use of electron beam lithography in order to obtain such patterns. Electron beam lithography is a method of drawing each pattern with an electron beam, and is poorly productive and impractical.
PTL 2 also describes removal of aluminum with a chlorine plasma. Typically, such etching results in adhesion of a chloride on side walls of the aluminum patterns. Commercially available wet etching liquids (for example, SST-A2 available from Tokyo Ohka Kogyo Co., Ltd.) can remove the chloride. However, these kinds of liquid medicines that react with aluminum chloride also react with aluminum, although at a low etching rate. Therefore, it is difficult to realize the desired pattern shape with the described method.
As another method, PTL 2 also describes a method of depositing aluminum on a patterned photoresist by oblique film formation and removing the photoresist (paragraphs [0045] and [0047]). However, it is considered that this method also needs to deposit aluminum on the substrate surface to some degree in order to obtain close adhesiveness between the substrate and aluminum. However, this means that the shape of the deposited aluminum films is different from the prolate sphere, which is the suitable shape described in paragraph [0015] and includes a prolate ellipsoid. It is also described in paragraph [0047] that an excessive deposit is removed by anisotropic etching perpendicular to the surface. In order to obtain the function as the polarizing plate, shape anisotropy of aluminum is extremely important. Accordingly, it is considered necessary to adjust the amounts of aluminum to be deposited on the resist portions and the substrate surface such that a desired shape can be obtained by etching. However, it is considered extremely difficult to obtain this control on a size condition described in paragraph [0047], i.e., 0.05 micrometers, which is equal to or less than submicron. It is doubted that this is suitable as a producing method having a high productivity. Further, as a property of the polarizing plate, a high transmittance is required in the transmission axis direction. Typically, when glass is used as the substrate, some percent of reflection from the glass interface is unavoidable, and a high transmittance is difficult to obtain.
PTL 3 describes a polarizing plate obtained by oblique deposition. This method for obtaining a polarizing property by producing minute columnar structures by oblique deposition of materials that are transparent and opaque to the wavelengths in the range used. Unlike PTL 2, minute patterns can be obtained with a simple method. Therefore, this method can be considered a highly productive method, but also has a problem. The aspect ratio of the minute columnar structures to be formed of the material opaque to the range used, the intervals between the individual minute columnar structures, and linearity of the individual minute columnar structures are important factors for obtaining a good polarizing property, and should be intentionally controlled from the viewpoint of the repeatability of the property. However, this method utilizes a phenomenon that a columnar structure is obtained by to-be-vapor-deposited particles to come flying next not being deposited on the shadow portion of an initially deposited layer of vapor-deposited particles. Therefore, it is difficult to intentionally control the factors described above. As a method for improving this, a method of providing polishing scars on the substrate by a rubbing treatment before vapor deposition is described. However, the particle diameter of the vapor-deposited film is typically about some tens of nanometers at the maximum, and it is necessary to intentionally produce a pitch equal to or less than submicron by polishing in order to control the anisotropy of such particles. However, the limit of, for example, a common polishing sheet is about submicron, and it is not easy to produce such minute polishing scars. Further, as described above, the resonance wavelength of Al particles is largely dependent on the surrounding refractive index. In this case, the combination of the transparent and opaque materials is important. However, PTL 3 does not describe a combination for obtaining a good polarizing property in a visible light range. Further, like PTL 2, when glass is used as the substrate, some percent of reflection from the glass interface is unavoidable.
NPL 2 describes a polarizing plate called LAMIPOL used for infrared communication. This has a laminated structure formed of Al and SiO2. According to this document, an extremely high extinction ratio is exhibited. NPL 3 describes that use of Ge instead of Al to be responsible for light absorption of LAMIPOL can realize a high extinction ration for a wavelength of 1 micrometer or less. Furthermore, from FIG. 3 of the same material, it is expected that a high extinction ratio can also be obtained with Te (tellurium). As can be understood, LAMIPOL is an absorptive polarizing plate that can obtain a high extinction ratio. However, because the thickness of the laminated layers of a light absorbing material and a transmissive material is the size of the light receiving surface, it is not suitable as a polarizing plate for a projector that needs to have a size of some centimeters square.
PTL 4 discloses a polarizing plate obtained by combining a wire grid structure and an absorbing film. When a metal or a semiconductor film is used as the absorbing film, the reflectance for a specific range can be reduced by adjusting the thickness of a dielectric film between the material, the wire grid, and the absorbing film, because the optical property of the material is strongly influential. However, it is difficult to realize this in a wide wavelength range.
Use of Ta and Ge that have a high absorbency makes it possible to broaden the range, but makes absorbency in the transmission axis direction high at the same time. This reduces the transmittance in the transmission axis direction, which is the important property as the polarizing plate.
Accordingly, currently, it is required to provide an inorganic polarizing plate having an excellent polarizing property and a method for producing the same.